1. Field of the Invention
The present invention relates to a fuel cell for generating electric power by an electrochemical reaction between hydrogen and oxygen.
2. Description of the Related Art
Recently much attention has been focused on fuel cells that feature not only high energy conversion efficiency but also no hazardous substance produced by the electricity-generating reaction. Known as one of such fuel cells is the polymer electrolyte fuel cell which operates at temperatures below 100° C.
A polymer electrolyte fuel cell, which has a basic structure of a solid polymer electrolyte membrane disposed between a fuel electrode and an air electrode, generates power through an electrochemical reaction as described below by supplying a fuel gas containing hydrogen to the fuel electrode and an oxidant gas containing oxygen to the air electrode.Fuel electrode: H2→2H++2e−  (1)Air electrode: (½)O2+2H++2e−→H2O  (2)
An anode and a cathode have each a stacked structure of a catalyst layer and a gas diffusion layer. And a fuel cell is composed of catalyst layers of the respective electrodes disposed counter to each other in such a manner as to support a solid polymer membrane therebetween. The catalyst layer is a layer of a catalyst or carbon particles supporting a catalyst bound together by an ion-exchange resin. The gas diffusion layer serves as a passage for the oxidant gas or the fuel gas.
At the anode, the hydrogen contained in the supplied fuel is decomposed into hydrogen ions and electrons as expressed in the above formula (1). Of them, the hydrogen ions travel inside the solid polymer electrolyte membrane toward the air electrode, whereas the electrons travel through an external circuit to the air electrode. At the cathode, on the other hand, the oxygen contained in the oxidant gas supplied thereto reacts with the hydrogen ions and electrons having come from the fuel electrode to produce water as expressed in the above formula (2). In this manner. the electrons travel from the fuel electrode toward the air electrode in the external circuit, so that the electric power is extracted therefrom.
When the fuel cell is stopped with a cessation of the supply of the fuel gas to the anode, air begins to mix into the gas on an anode side. If the fuel cell is started again in this state, protons will be conducted from the anode to the cathode through the electrolyte membrane on an upstream side where the density of the fuel gas is high. On a downstream side, however, where the density of the fuel gas is low due to the mixing of air, a reaction as expressed in the formula below progresses at the cathode, and a reverse current flows with protons conducted from the cathode to the anode.
More specifically, as illustrated in FIG. 1, on the upstream side of the reaction gas, reactions as expressed in formulas (3) and (4) below take place the same way as in ordinary cell reaction at an anode 2 and a cathode 4, respectively, which supports an electrolyte membrane 6 in between. On an exit side (downstream side), on the other hand, reactions as expressed in formulas (5) and (6) below take place at the anode 2 and the cathode 4, respectively, and a reverse current is produced. As a result of the reaction of formula (6) at the cathode 4 on the exit side, oxidation and corrosion of carbon particles supporting a catalyst and an ion-exchange resin, both used in the cathode 4, progress thereby deteriorating the electronic performance and shortening the life of the fuel cell.
Upstream side:Anode: H2→2H++2e−  (3)Cathode: O2+4H++4e−→2H2O  (4)
Downstream side:Anode: O2+4H++4e−→2H2O  (5)Cathode: C+2H2O→CO2+4H++4e−  (6)